Import mxnet and tensorflow only if explicitly enabled

[adrianeboyd]

Description

Import mxnet and tensorflow only if explicitly enabled

Types of change

?

Checklist

• I confirm that I have the right to submit this contribution under the project's MIT license.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[adrianeboyd]

This was the most minimal way I could think of to do this for thinc v8.

I mainly think that nearly no one is using thinc with mxnet or tensorflow so this isn't going to be very breaking either way.

I don't know how/if you could add deprecation warnings to v8.1 to prepare users for the change. Again, I think it's hardly anyone. I doubt many people are using this ancient version of tensorflow and I guess I don't really know, but I don't think mxnet ever had a large user base.

[danieldk]

We should probably decide if we want to remove this functionality from Thinc 9 and if so, add warnings to the enable_ functions.

[rmitsch]

LGTM. Maybe @danieldk wants to give it a final once-over?